Vision: Support Migrating Encoded Calls

[apopiak]

When changing the encoding of calls (e.g. changing pallet indices or extrinsic ordering) we need to migrate encoded calls (e.g. in Scheduler or Democracy) to the new format.
Currently we can only hope that they are not affected by runtime upgrades or clear them in case of incompatibility.

[apopiak]

relates to #353

[bkchr]

Hmm what support do you want to have for this? You need the old Call enum to decode the data and than you will need to convert this into the new Call enum.

[apopiak]

Jep, we'd want some (at least moderately) easy way to get the deprecated Call enum.

[shawntabrizi]

We entertained the idea of using a call hash instead of a call enum, but this would result in a larger payload size for all transactions.

A solution would just be to allow us to control the call index of the extrinsics in a pallet. We are already able to control the module index too, so together, things would remain deterministic and non-breaking.

[apopiak]

You would still (on top of that) want to be able to determine removed indices as well as changed ones to migrate properly.